Curable organopolysiloxane compositions that cure by the hydrosilylation reaction are used in optical semiconductor devices such as photocouplers, light-emitting diodes, solid-state imaging elements, and so forth as, for example, protective coatings and sealants for optical semiconductor elements. Since an optical semiconductor element emits or receives light, these protective coatings and sealants for optical semiconductor elements must not absorb or scatter light.
As a consequence, Patent References 1 to 5 provide hydrosilylation reaction-curable organopolysiloxane compositions that, through the use of organopolysiloxane having a high phenyl content, are able to form cured products that exhibit a high refractive index and a high optical transmittance.
However, the present inventors noted that these curable organopolysiloxane compositions had the following problems: their cured products do not always exhibit a high optical transmittance; they exhibit a poor durability of adhesion for the semiconductor element or optical semiconductor element, lead frame, and package in contact with the composition during its cure, resulting in facile detachment. With regard to semiconductor devices provided with a semiconductor element that has been coated or sealed with these curable organopolysiloxane compositions or an optical semiconductor device provided with an optical semiconductor element that has been coated or sealed with these curable organopolysiloxane compositions, the present inventors also noted the problems of a poor durability of adhesion and an unsatisfactory reliability.
A curable organopolysiloxane composition that solves these problems is disclosed in Patent Reference 6. However, the present inventors noted the following problem with this curable organopolysiloxane composition: because an essential component of this composition is a radical copolymer-type adhesion promoter, when an optical semiconductor element coated or sealed with a cured product from this composition is subjected to long-term use at high temperatures, this cured product undergoes yellowing and the optical transmittance declines.